Improve the variable_changed signals and their info dictionaries

[Jowan-Spooner]

Improved documentation and implementation of Dialogic.VAR.variable_was_set and Dialogic.VAR.variable_changed.

The variable_was_set info now will consitently contain these keys:

## `variable`  | [type String] | The name of the variable that is getting changed. [br]
## `orig_value`| [type Variant]| The value that [variable] had before. [br]
## `new_value` | [type Variant]| The value that [variable] has after the change (the result). [br]
## `value`     | [type Variant]| The value that the variable is changed by/to. [br]
## `value_str` | [type String] | Whatever has been given as the value (not interpreted, so a variable is just a string).[br]

• slightly compatibility breaking